1. Field of the Invention
The present invention relates to a manual switching valve for enabling switching of a flow passage state of a fluid under the displacement action of a valve plug.
2. Description of the Related Art
Heretofore, a manual switching valve has been known in which a valve plug is disposed displaceably inside of a body equipped with a passage therein through which a fluid flows, and wherein a communication state of the fluid is switched by seating and unseating the valve plug on a valve seat that faces the passage. Generally, in such a manual switching valve, when the communication state of the fluid is changed, an operator rotates a handle that makes up an operating element, whereby the valve plug is displaced along an axial direction for seating and unseating the valve plug with respect to the valve seat, and thus switching between valve closed and valve open states.
However, for example when the manual switching valve is placed in a valve closed state, in the event that the operator mistakenly rotates the handle with an excessive rotational torque, the valve plug is displaced greater than a predetermined amount toward the side of the valve seat, such that the valve plug presses against the valve seat with an excessive force. In this case, there is a concern that the valve plug and/or the valve seat may be damaged by the excessive force (torque), to such an extent that the durability thereof is lowered. For this purpose, a manual switching valve is known in which contact at an excessive force between the valve plug and the valve seat can be prevented.
For example, as disclosed in Japanese Laid Open Patent Publication No. 2005-344918, in such a manual switching valve, in the event that a valve closed state is made by seating the valve plug onto the valve seat of the valve main body, a slide nut, which is threadedly engaged with a handle, is rotated by turning the handle, and a piston that is threaded into an interior part of the slide nut is displaced toward the valve seat so that the valve plug is seated on the valve seat. In addition, in the case that the handle is rotated further, the slide nut, which is threadedly engaged between the handle and the piston, is displaced upwardly, whereupon the slide nut becomes detached from the piston, and as a result the threaded state with respect to the piston is released. Owing thereto, the handle is simply rotated idly and the rotational force imposed on the handle is not transmitted to the piston, such that further displacement of the valve plug toward the valve seat is prevented.
However, in this conventional technique, although a structure is provided for releasing the threaded engagement of the piston and the slide nut in order to interrupt the rotational force from the handle, when the manual valve is used again, it is necessary to reestablish the threaded engagement between the piston and the slide nut. For this purpose, a spring is provided separately, wherein the slide nut is urged toward the piston and guided into threaded engagement therewith by the spring. In this manner, with the manual valve according to the conventional technique, the structure for enabling prevention of further additional pressing force of the valve plug with respect to the valve seat upon closing the valve is considerably complex.
Further, such a structure only prevents application of excessive force on the valve plug and the valve seat when the valve is completely closed. The structure cannot prevent the application of an excessive force on the valve plug when the valve is completely open, in a state in which the valve plug is separated away from the valve seat.